Silently this summer, political cartographers are drawing lines on maps. Should we care? So much else is more pressing than the efforts of boundary commissions to turn 650 old constituencies into 600 new ones, containing more or less equal numbers of registered voters. Yet this process will shape the next election. It will also produce shrieks of pain and perhaps that much sought-after crisis that could crack the coalition.

Imagine that you were the Liberal Democrat MP for West Woldshire, popular in your patch (even now), which you keep flooding with Focus leaflets that no longer contain pictures of Nick Clegg, in the hope that local loyalty will carry you back into the Commons in 2015. You've worked there for years, building your reputation. But next month, in a peculiar sort of judgment day, you may learn from the Boundary Commission for England that your seat is for the chop: bits of West Woldshire are to be incorporated with the rolling acres of neighbouring East Shireworth, which has always voted Tory. There's a quickfire appeals process, but not much chance of overturning the proposal before the final judgment is made. How loyal – given your parliamentary career is likely to end soon – do you feel to the coalition now?

Private battles such as this will be played out over the next few months. The Commons must approve the changes before October 2013. It's easy to imagine disgruntled Tories and Lib Dems combining forces with Labour to vote down the plan. So it matters that the process is understood and seen to be fair. Neither goal can be said to have been achieved yet.

Because of a deal reached in the coalition negotiations, trading a reduction in the size of the Commons for a vote on electoral reform, almost every seat in the UK is being changed somewhat, many by a lot. A reasonable number will cease to exist at all. It's a political game of musical chairs, being played to Conservative advantage, since some of the unevenness that made Tory votes less powerful than Labour ones at the last election will be ironed out.

Estimates of the impact vary. One analysis, set out last week by the Conservative party psephologist Rob Hayward, calculates that his party will lose 15 seats as parliament shrinks, Labour 25 and other parties a combined 10. Another, by the analyst Lewis Baston, suggests the Lib Dems would suffer much more and Labour less. The truth is that no one will know until the four boundary commissions set out their plans. Even then, it will be hard to calculate how the new seats would have voted had they been in place at the last election, and just as tricky, in many cases, to work out which candidate belongs where. There will be some high-profile spats: the chancellor, George Osborne, will be looking for a revised home in Cheshire, and the shadow chancellor, Ed Balls, may have to do the same in Yorkshire. Sheffield – which loses the equivalent of nearly half a seat to its peripheries – may turf out Clegg.

Both Labour and the Conservatives have private plans to offer rejected MPs sanctuary elsewhere (or at least those who the whips wish to reward). It's another reason why reform of Lords patronage looks unlikely. The Lib Dems, meanwhile, say reports of their problems are overdone. The party thinks their MPs will root themselves in the new seats through local campaigning. I think they are over-optimistic.

What's surprising (and has not entirely sunk in outside Westminster) is the speed. Draft proposals will come this autumn, followed by public hearings at which people can make short statements, but not – as before – deploy barristers. The commissions may change their minds but will be under no pressure to do so, and there will be no process of further hearings on their final proposals.

Is this a fix? Labour thinks so: "gerrymandering", the party screamed as the bill went through parliament. But there is nothing unfair about equal-sized seats (the fifth of the Chartists' six demands) or a quicker, less partisan, process for settling them. The boundary commissions are accepted by everyone to be independent.

Nor will new boundaries stop Labour MPs winning with fewer votes than Tory ones. Higher turnout in Tory areas is the main reason for that. It's true that the boundaries are being decided on the registered electorate and that in towns people may be less likely to register. But this is how it has always been done. Wales will lose a disproportionate number of seats, but that is a consequence of previous Welsh over-representation. Past unfairness has at least as much a part to play in shaping what will happen as newly engineered ones.

Still, it cannot be denied that a reason David Cameron has pushed so hard for this change is because he will benefit from it, or that by cutting the number of MPs, while keeping the number of ministers constant, the Commons will be weakened. When the proposals for England come out next month, the Lib Dems will be given another reason to doubt the coalition. In politics, as in sport, it's often a row about rules rather than a test of the teams' mettle that proves the most bitter contest. Expect angry shouting from within closed committee room doors and even the possibility of a walkout.